Fermentation is a biological process that uses yeasts to convert feedstocks such as grains and cellulosic plant matter into ethanol. Fermentation processes can be used to generate potable ethanol in the form of liquors and other alcoholic beverages as well as to generate industrial or fuel ethanol. Fermentation is often coupled with distillation processes that isolate the ethanol from the fermentation mixture. While the fermentation and distillation processes can effectively generate and isolate ethanol, the remaining liquids and solids can pose a problem for operators of ethanol production facilities. The remaining liquids and solids must either be disposed of or undergo further processing to generate viable commodities. Both disposal and/or processing of these remaining liquids and solids can increase the costs and complexity of ethanol production. For example, while further processing of the remaining liquids can render them suitable to be reused at least in part in the fermentation process, this further processing can be costly and there are limits to the amount of liquid that can be recycled. Likewise, there are increased costs and limitations to further processing the remaining solids into useful products.
In general, in conventional ethanol production, a prepared and/or processed mixture of feedstock is fermented to generate ethanol. An “ethanol” fraction is further processed by distillation to isolate ethanol. The fraction remaining after distillation is known as “whole stillage” and is separated into a more solid fraction known as “wet grains” and a more liquid fraction known as “thin stillage.” Wet grains can contain from about 7-17% solids and can be sold as “wet distillers grains” or can be dried to be sold as “dried distillers grains” (“DDG”) or “dried distillers grains with solubles” (“DDGS”). While the separation process removes some of the total solids (TS) and total suspended solids (TSS) that are found in the whole stillage, substantial amounts remain in the thin stillage. The thin stillage can also contain unfermented feedstock components such as hemicellulose, yeasts, and other solids typically associated with structural components of the feedstock (e.g., biotins, dextrans, and other similar components). Thin stillage can be recycled as “backset” into the fermentation step to take the place of some of the water that is added to the prepared and/or processed mixture of feedstock. However, the ratio of thin stillage that can be used as backset is limited in part because of the TS, TSS, solids, and unfermentable components found in the thin stillage. Thin stillage that is not used as backset needs to be further processed by a relatively costly evaporation process to separate water from any remaining solids.
The majority of water that makes up the recipe/mix for a new fermentation (cooking) batch is made up of 1) evaporation condensate (majority); 2) CO2 scrubber water (smallest amount); and 3) backset. If an ethanol fermentation plant had enough evaporation capacity it would send nearly all evaporation condensate back to begin the cooking process. But since ethanol fermentation plants do not have sufficient evaporation capacity, primarily because the evaporation process is very expensive, ethanol fermentation plants return a fraction of the thin stillage untreated as “backset” to the fermentation (cook) process. The percentage of backset that can be used is limited for the reasons listed.
Therefore, although conventional ethanol production provides some processes to recycle the liquids and solids remaining after the fermentation step, these processes are not without their shortcomings. Some examples of shortcomings include the limited ratio of thin stillage that can be used as backset and the cost of further processing by evaporation. Accordingly, it would be an improvement in the art to augment or even replace current technologies with other techniques.